in examle ``` use std::cell::RefCell; use std::rc::Rc; struct MyStruct { s: u32 } fn main() { let mut x = Rc::new(RefCell::new(MyStruct{ s: 5u32 })); let y = x.clone(); x.borrow_mut().s = 6; println!("{}", x.borrow.s); } ``` we must use `borrow()` instead of just `borrow`